Modern jet aircraft, particularly those employed in the Navy require improved hook point material in order to arrest the craft during carrier and field carrier landing practice. In arresting the craft, friction and thermal loading results from engagement and as the hook point engages and slides when mating the arresting cable. Current materials and protective surfaces are not easily manufactured and associated processing is time consuming and costly. Therefore, there have been attempts to develop new innovative alloys and processes that will be more cost and energy effective while improving performance in arresting modern aircraft. Of particular interest are the wear and corrosion resistance of surfaces that mate and slide while the arresting gear hook point engages and arrests the aircraft. It would be beneficial then to develop a material and processing that would not require external processing to protect or minimize the engaging surface against wear and corrosion. The characteristics of the alloy should also satisfy internal properties required and characteristic of stainless steel alloys.
One goal of the present invention is to develop a process for an improved performance steel using high temperature carburizing or nitriding for improved wear resistance with improved corrosion resistance in respect to stainless steel alloys without compromising internal base properties of the steel. To this end, a primary objective was to develop a carburization process that would provide a hard, wear resistant surface that would in addition provide the corrosion resistance of a martensitic stainless steel. This is accomplished by using the inherent protection of the chromium to enhance the wear resistance by formation of protective carbides, in addition to the properties afforded by martensitic formation of the base surface without sacrifice of the corrosion resistance provided by the chromium addition to the martensitic stainless steel.
Stainless steels may be classified by their crystalline structure into three main types: Austenitic, Ferritic and Martensitic. Martensitic steels are steels comprising a composition of iron, 12% chromium, and 0.12% carbon. They may be tempered and hardened. Martensitic steel possesses great hardness, but has reduced toughness and is brittle; so few steels are fully hardened. Martensitic steel is formed by the rapid cooling (quenching) of austenite, which traps carbon atoms that do not have time to diffuse out of the crystal structure. This martensitic reaction begins during cooling when the austenite reaches the martensitic steel start temperature (Ms) and the parent austenite becomes mechanically unstable. At a constant temperature below Ms, a fraction of the parent austenite transforms rapidly, and then no further transformation will occur. When the temperature is decreased, more of the austenite transforms to martensite. Finally, when the martensite finish temperature (Mf) is reached, the transformation is complete. Martensite can also form by application of stress (this property is frequently used in toughened ceramics like yttrium—stabilized zirconium and in special steels like TRIP steels (i.e. transformation induced plasticity steels)). Thus, martensite can be thermally—induced or stress—induced.
Austenite (or gamma phase iron) is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element, such as ferric carbide or carbon in iron, used in making corrosion-resistant steel. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1,000° K. (730 ° C.); other alloys of steel have different eutectoid temperatures. Its face-centered cubic (FCC) structure allows it to hold a high proportion of carbon in solution. As it cools, this structure either breaks down into a mixture of ferrite and cementite (usually in the structural forms pearlite or bainite), or undergoes a slight lattice distortion known as martensitic transformation. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel. Quenching (to induce martensitic transformation), followed by tempering (to break down some martensite and retained austenite), is the most common heat treatment for high-performance steels.
The addition of certain other metals, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. On the other hand, such elements as silicon, molybdenum, and chromium tend to de-stabilize austenite, raising the eutectoid temperature (the temperature where two phases, ferrite and cementite, become a single phase, austenite).
Austenite can contain far more carbon than ferrite, between 0.8% at 723° C. and 2.08% at 1148° C. Thus, above the critical temperature, all of the carbon contained in ferrite and cementite (for a steel of 0.8% C) is dissolved in the austenite.
One of the differences between the two phases is that martensite has a body centered tetragonal crystal structure, whereas austenite has a face center cubic (FCC) structure. The transition between these two structures requires very little thermal activation energy because it is a martensitic transformation, which results in the subtle but rapid rearrangement of atomic positions, and has been known to occur even at cryogenic temperatures. Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume.
Carburization is the introduction of additional carbon to the surface of a steel part in order to effect surface hardening. In gaseous carburizing, an endothermic gas, which contains carbon monoxide and hydrogen, is used as a carrier gas to displace the air in the furnace. A hydrocarbon-containing gas such as natural gas, propane or butane is added to the endothermic gas in varying quantities. The carbon monoxide from the endothermic gas and the hydrocarbon react to form nascent carbon atoms, which in turn combine with the iron in the steel to form iron carbide. The iron carbide provides the hard surface.
The steel is exposed to the carburizing atmosphere at high temperatures, e.g. temperatures in the austenitic range for the steel in question, for a predetermined time to achieve the desired depth of carbon penetration into the steel surface. This depth is called the depth of the case. Carburized or case hardened steel has many important uses because of its desirable properties. The case provides extreme hardness at the surface while the inner portion, or core, beyond the case is relatively soft and ductile. Therefore, case hardened steel has excellent wear properties in combination with the toughness of the core.
In conventional gas carburizing furnaces, a carburizing atmosphere is force circulated by a fan system over the steel in the chamber at atmospheric pressures. Usually, a hydrocarbon gas, such as is found in natural gas, is utilized in combination with a carrier gas, such as an endothermic gas, as the carburizing atmosphere. The carburizing atmosphere is circulated in the furnace for a predetermined time and under predetermined conditions to carburize the steel. The various ramifications and modifications of this technique are well known to those skilled in the art.
Carburization of steel involves a heat treatment of the metallic surface using a gaseous, liquid, solid or plasma source of carbon. Early carburization techniques used a direct application of charcoal packed onto the metal (initially referred to as case hardening or Kolsterizing), but modern techniques apply carbon-bearing gases or plasmas (such as acetylene or methane). The process depends primarily upon gas composition, temperature, and the deposition and diffusion times, which must be carefully controlled. For applications where greater control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.
Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics (such as wear and corrosion resistance, hardness and load bearing capacity, in addition to quality-based variables) of various metals, notably stainless steels. The process is used because it is environmentally friendly (in comparison to gaseous or solid carburization). However, the process is more expensive and requires a “line-of-sight” procedure to carburize or nitride the steel surfaces.
The process of carburization works via the implantation of carbon atoms into the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution (dissolved within the metal crystalline matrix (this normally occurs at lower temperatures) or react with the host metal to form ceramic carbides (normally at higher temperatures, due to the higher mobility of the host metals' atoms). Both of these mechanisms strengthen the surface of the metal, the former by causing lattice strains by virtue of the atoms being forced between those of the host metal and the latter via the formation of very hard particles that resist abrasion. However, each different hardening mechanism leads to different solutions to the initial problem. The former mechanism, known as solid solution strengthening, improves the host metals' resistance to corrosion while increasing the alloys hardness. The latter, known as precipitation strengthening, greatly improves the hardness of the alloy but normally to the detriment of the host metals' corrosion resistance. Engineers using plasma carburization must decide which of the two mechanisms matches their needs.
There are different types of elements or materials that can be used to perform this process, but these mainly consist of high carbon content material. A few typical hardening agents include carbon monoxide gas (CO), sodium cyanide (NaCN) and barium chloride (BaCl2), or hardwood charcoal. In gas carburizing, the CO is derived from a propane or natural gas source. In liquid carburizing, the CO is derived from a molten salt composed mainly of sodium cyanide and barium chloride. In pack carburization, the carbon monoxide source comes from coke or hardwood charcoal.
In pack carburizing, the work piece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickel/chromium alloy and sealed at all openings with fire clay. In gas nitriding the donor is a nitrogen-rich gas, usually ammonia (NH3), which is why it is sometimes known as ammonia nitriding. When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses from the surface into the core of the material. This process has been around for nearly a century though only in the last few decades has there been a concentrated effort to investigate the thermodynamics and kinetics involved.
U.S. Pat. No. 4,386,973 to Herring et. al. discloses and claims a process for carburizing steel comprising carburizing said steel under vacuum in a vacuum furnace in the presence of a hydrocarbon carburizing agent while utilizing as a carrier gas an aliphatic alcohol having 1 to 4 carbon atoms. The carburizing agent is supplied during said carburizing by cyclically reducing the pressure in the furnace followed by the addition of hydrocarbon carburizing agent to the furnace to replenish gas removed in the pressure reduction step.
The carburization process per se then, is not new technology. The standard process simply introduces carbon to the surface of a steel and relies on diffusion to transport the carbon internally from the surface into the interior. There are many methods developed to accomplish this using solid state carburization, such as pack carburization, traditional gas carburization and more recently, partial pressure gas carburization in vacuum furnaces. The later process has recently been successful in the carburization of traditional low alloy and tool steels. However, owing to the problem of grain growth, the carburization temperature is usually limited to the range of 845° C. to 950° C. This requires long periods of time in order to permit sufficient diffusion to occur so as to obtain the required depth of carburization to protect against wear.